287 research outputs found
Carrier multiplication yields in CdSe and CdTe nanocrystals by transient photoluminescence
Engineering semiconductors to enhance carrier multiplication (CM) could lead
to increased photovoltaic cell performance and a significant widening of the
materials range suitable for future solar technologies. Semiconductor
nanocrystals (NCs) have been proposed as a favourable structure for CM
enhancement, and recent measurements by transient absorption have shown
evidence for highly efficient CM in lead chalcogenide and CdSe NCs. We report
here an assessment of CM yields in CdSe and CdTe NCs by a quantitative analysis
of biexciton and exciton signatures in transient photoluminescence decays.
Although the technique is particularly sensitive due to enhanced biexciton
radiative rates relative to the exciton, kradBX > 2 kradX, we find no evidence
for CM in CdSe and CdTe NCs up to photon energies E > 3 Eg, well above
previously reported relative energy thresholds.Comment: 9 pages, 6 figure
Final Report for DE-FG36-08GO18007 "All-Inorganic, Efficient Photovoltaic Solid State Devices Utilizing Semiconducting Colloidal Nanocrystal Quantum Dots"
We demonstrated robust colloidal quantum dot (QD) photovoltaics with high internal quantum efficiencies. In our structures, device durability is derived from use of all-inorganic atmospherically-stable semiconducting metal-oxide films together with QD photoreceptors. We have shown that both QD and metal-oxide semiconducting films and contacts are amenable to room temperature processing under minimal vacuum conditions, enabling large area processing of PV structures of high internal efficiency. We generated the state of the art devices with power conversion efficiency of more than 4%, and have shown that efficiencies as high as 9% are achievable in the near-term, and as high as 17% in the long-term
Morphology of passivating organic ligands around a nanocrystal
Semiconductor nanocrystals are a promising class of materials for a variety
of novel optoelectronic devices, since many of their properties, such as the
electronic gap and conductivity, can be controlled. Much of this control is
achieved via the organic ligand shell, through control of the size of the
nanocrystal and the distance to other objects. We here simulate ligand-coated
CdSe nanocrystals using atomistic molecular dynamics, allowing for the
resolution of novel structural details about the ligand shell. We show that the
ligands on the surface can lie flat to form a highly anisotropic 'wet hair'
layer as opposed to the 'spiky ball' appearance typically considered. We
discuss how this can give rise to a dot-to-dot packing distance of one ligand
length since the thickness of the ligand shell is reduced to approximately
one-half of the ligand length for the system sizes considered here; these
distances imply that energy and charge transfer rates between dots and nearby
objects will be enhanced due to the thinner than expected ligand shell. Our
model predicts a non-linear scaling of ligand shell thickness as the ligands
transition from 'spiky' to 'wet hair'. We verify this scaling using TEM on a
PbS nanoarray, confirming that this theory gives a qualitatively correct
picture of the ligand shell thickness of colloidal quantum dots.Comment: 17 Pages, 9 Figure
Revealing single emitter spectral dynamics from intensity correlations in an ensemble fluorescence spectrum
We show that the single emitter linewidth underlying a broadened ensemble
emission spectrum can be extracted from correlations among the stochastic
intensity fluctuations in the ensemble spectrum. Spectral correlations can be
observed at high temporal and spectral resolutions with a cross-correlated pair
of avalanche photodiodes placed at the outputs of a scanning Michelson
interferometer. As illustrated with simulations in conjunction with
Fluorescence Correlation Spectroscopy, our approach overcomes ensemble and
temporal inhomogeneous broadening to provide single emitter linewidths, even
for emitters under weak, continuous, broadband excitation.Comment: 9 pages, 5 figure
Recommended from our members
Generalized Kasha's Model: T-Dependent Spectroscopy Reveals Short-Range Structures of 2D Excitonic Systems
A Low Reabsorbing Luminescent Solar Concentrator Employing π-Conjugated Polymers
A highly efficient thin-film luminescent solar concentrator (LSC) utilizing two π-conjugated polymers as antennae for small amounts of the valued perylene bisimide Lumogen F Red 305 is presented. The LSC exhibits high photoluminescence quantum yield, low reabsorption, and relatively low refractive indices for waveguide matching. A Monte Carlo simulation predicts the LSC to possess exceptionally high optical efficiencies on large scales.National Science Foundation (U.S.) (Graduate Research Fellowship Program (Grant No. 1122374))Eni S.p.A. (Firm) (Eni-MIT Solar Frontiers Alliance
Carrier multiplication yields in PbS and PbSe nanocrystals measured by transient photoluminescence
We report here an assessment of carrier multiplication (CM) yields in PbSe
and PbS nanocrystals (NCs) by a quantitative analysis of biexciton and exciton
dynamics in transient photoluminescence decays. Interest in CM, the generation
of more than one electron and hole in a semiconductor after absorption of one
photon, has renewed in recent years because of reports suggesting greatly
increased efficiencies in nanocrystalline materials compared to the bulk form,
in which CM was otherwise too weak to be of consequence in photovoltaic energy
conversion devices. In our PbSe and PbS NC samples, however, we estimate using
transient photoluminescence that at most 0.25 additional e-h pairs are
generated per photon even at energies hv > 5Eg, instead of the much higher
values reported in the literature. We argue by comparing NC CM estimates and
reported bulk values on an absolute energy basis, which we justify as
appropriate on physical grounds, that the data reported thus far are
inconclusive with respect to the importance of nanoscale-specific phenomena in
the CM process.Comment: 10 pages, 7 figure
Improved performance and stability in quantum dot solar cells through band alignment engineering
Solution processing is a promising route for the realization of low-cost, large-area, flexible and lightweight photovoltaic devices with short energy payback time and high specific power. However, solar cells based on solution-processed organic, inorganic and hybrid materials reported thus far generally suffer from poor air stability, require an inert-atmosphere processing environment or necessitate high-temperature processing [superscript 1], all of which increase manufacturing complexities and costs. Simultaneously fulfilling the goals of high efficiency, low-temperature fabrication conditions and good atmospheric stability remains a major technical challenge, which may be addressed, as we demonstrate here, with the development of room-temperature solution-processed ​[ZnO over ​PbS] quantum dot solar cells. By engineering the band alignment of the quantum dot layers through the use of different ligand treatments, a certified efficiency of 8.55% has been reached. Furthermore, the performance of unencapsulated devices remains unchanged for over 150 days of storage in air. This material system introduces a new approach towards the goal of high-performance air-stable solar cells compatible with simple solution processes and deposition on flexible substrates.National Science Foundation (U.S.)Hertz FoundationSamsung Advanced Institute of TechnologyNational Institutes of Health (U.S.) (Massachusetts Institute of Technology. Laser Biomedical Research Center. Contract 9-P41-EB015871-26A1)National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (MIT Center for Materials Science and Engineering. Award DMR-08-19762
Objective, comparative assessment of the penetration depth of temporal-focusing microscopy for imaging various organs
Temporal focusing is a technique for performing axially resolved widefield multiphoton microscopy with a large field of view. Despite significant advantages over conventional point-scanning multiphoton microscopy in terms of imaging speed, the need to collect the whole image simultaneously means that it is expected to achieve a lower penetration depth in common biological samples compared to point-scanning. We assess the penetration depth using a rigorous objective criterion based on the modulation transfer function, comparing it to point-scanning multiphoton microscopy. Measurements are performed in a variety of mouse organs in order to provide practical guidance as to the achievable penetration depth for both imaging techniques. It is found that two-photon scanning microscopy has approximately twice the penetration depth of temporal-focusing microscopy, and that penetration depth is organ-specific; the heart has the lowest penetration depth, followed by the liver, lungs, and kidneys, then the spleen, and finally white adipose tissue.National Institutes of Health (U.S.) (NIH-5-P41-EB015871-27)National Institutes of Health (U.S.) (DP3-DK101024 01)National Institutes of Health (U.S.) (1-U01-NS090438-01)National Institutes of Health (U.S.) (1-R01-EY017656 -0,6A1)National Institutes of Health (U.S.) (1-R01-HL121386-01A1)National Institutes of Health (U.S.) (NIH 5U54 CA151884-04)National Institutes of Health (U.S.) (9-P41-EB015871-26A1
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